It is well known that molded plastic foams (briefly, molded foams) are generally produced by one of the two basic techniques, namely a molding method employing expandable resin beads (for example, expandable polystyrene) and a molding method employing a liquid foaming material (for example, forming polyurethane). The former molding method, alias "in-mold foaming", comprises filling a metal mold with pre-expanded resin beads and introducing hot steam into the beads to cause the beads to expand further and thermally fuse together (See JP Publication H-3-56905).
Meanwhile, molded thermoplastic resin foams are known which have an integral outer skin of increased density (See JP Publication S-42-10752). They are produced by a process comprising the steps of blow-molding a skin-forming non-foamable parison in a blow-molding metal mold, cooling and allowing the molded parison to form a hollow member, taking it out from the mold, filling the hollow member with pre-expanded resin beads (hereinafter also referred to as "pre-expanded beads"), inserting a hot steam blowing pipe, feeding hot steam through the pipe to heat the pre-expanded beads for further expansion, and withdrawing the blowing pipe before the complete mutual fusion of the beads to occur to thereby provide a molded foam having a skin (See JP Publication S-62-19239, JP Publication S-58-10217 and J Utility Model Publication S-62-9073).
However, since this process includes two separate molding steps, namely one for molding a hollow member and one for molding a molded foam in the hollow parison, separate sets of apparatus including a metal molds are required, and, therefore, the whole producing process is complicated. Particularly, the cooling and solidification time required for the hollow member and the time required for mutual fusion of the pre-expanded beads and cooling and solidification thereof add up to a substantial length of time and, therefore, the productivity of the process is inevitably low. Furthermore, since the pre-expanded beads are introduced into the cooled hollow member, the bond between the hollow member and molded foam is insufficient.
To overcome these disadvantages, a method has been proposed which comprises pouring an urethane or other reactive foamable liquid material into the parison set in a closed blow-molding metal mold to bring the parison into intimate contact with the inside wall of the metal mold and causing the foamable liquid material to react and cure concurrently with the cooling and solidification of the parison under clamped condition (See JP Kokai S-58-116135 and JP Kokai H-3-293113).
However, because in this method the pressure required to cause the parison to conform to the inner wall of the blow-molding metal mold comes from from the pressure generated by the foaming of the foamable material, the method has a disadvantage that the pressure acting on the parison tends to be insufficient so that, particularly when the geometry of the inner wall is complicated, the parison cannot be fully brought into contact with the internal surfaces of the mold and the distribution of the degree of expansion of the parison is also found uneven.
Under the circumstances the inventors previously proposed a method for minimizing the molding cycle time by integrating an ordinary metal mold for blow-molding and a metal mold for in-mold foaming into one and the same mold, which method comprises blow-molding a parison in usual manner, feeding pre-expanded beads into the resulting hollow member, and heating the beads to cause further expansion and mutual fusion (JP Application H-3-252111).
Thus, in this method a pre-expanded bead filler is disposed in a retractable manner on the parting line of the blow-molding metal mold and a parison is blow-molded to form a hollow member while retaining the filler projected inwardly with respect to the internal surface of the mold, and then pre-expanded beads are introduced from the filler into the internal cavity of the hollow member. In this arrangement, the position of the filler is restricted to the parting line of the mold because it is difficult to make a filling hole in the hollow member with the filler and, therefore, the method cannot be applied to a large-sized or hinged product or in molding using a multi-cavity mold.